1. Field of the Invention
This invention relates to methods of and apparatus for correcting errors in binary data, and particularly, but not exclusively, to methods of and apparatus for correcting errors in digital television signals after recording and reproduction.
2. Description of the Prior Art
It is now quite common for analog signals, such as audio or television signals, to be put into digital form, for example using pulse code modulation, for transmission or for recording. It is also quite usual to add some check words to the coded data words before transmission or recording for use on reception or reproduction to detect and correct errors in the received or reproduced data words. Sometimes the check words are derived quite simply as, for example, in the case of simple parity check words and cyclic redundancy check words. However, such simple methods generally imply a relatively low level of error detection or correction capability, or alternatively that a relatively large number of check words are required to achieve a desired level of security against errors. As all such check words are additional to the data words, and therefore in a sense redundant, more sophisticated methods of generating the check words, which result in an improved error detection and correction capability without undue increase in the number of check words required are in use particularly for digital television signals where the amount of data involved means that even without the additional of redundant words, very high bit rates have to be used. Examples of more sophisticated methods which have been used for digital television signals are various so-called b-adjacent codes, of which the Reed-Solomon code is a particular example, and the Bose-Chaudhuri-Hocquenghem code.
A common problem with prior methods of error detection and correction using check words is that if an error occurs in a check word, for example, during transmission or during recording and reproduction, this may mean that error detection and correction using that check word is impossible, or alternatively wrong error detection and correction may occur. In the case of a digital television signal either of these eventualities may result in serious deterioration of a reproduced television picture.
Another problem is that while the use of such an error detection and correction method using check words may permit the detection and correction of up to several errors in the block of data words protected by those check words, the method does not provide adequate protection against a burst error involving a substantial sequence of data words and caused, for example, by a tape drop-out.
Our European patent application No. 83304289.8 discloses methods of correcting errors in binary data using check words which are derived in dependence not only on data words to be protected but also in dependence on other check words. The present invention is concerned with developments of those methods.
One object of the present invention is to provide a method of correcting errors in binary data and in which the problems referred to above are alleviated.
Another object of the present invention is to provide apparatus for correcting errors in binary data which are not only dependent on associated data words but also on associated check words.
Another object of the present invention is to provide a method of correcting errors in binary data in which errors in check words can be detected and in some cases corrected.
According to the present invention there is provided a method of correcting errors in binary data, the method comprising assembling the data into a plurality of data blocks, each said data block comprising a first plurality of data words and a second plurality of check words, each said check word being derived in dependence on all said data words in said data block and each other said check word in said data block, and further deriving a parity check block for a plurality of said data blocks.
According to the present invention there is also provided apparatus for correcting errors in binary data, the apparatus comprising means for assembling the data into a plurality of data blocks, each said data block comprising a first plurality of data words and a second plurality of check words, means for deriving each said check word in dependence on all said data words in said data block and each other said check word in said data block, and means for further deriving a parity check block for a plurality of said data blocks.
The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings.